Spot-welding guns are used to weld overlapping metal sheets, for example. Usually, such welding guns comprise air-operated or motor-driven servo welding guns capable of simplifying the controlling of the opening between a pair of electrode tips.
Generally, a motor-driven welding gun which is a motor-driven servo welding gun comprises a servomotor, a pair of electrode tips, and a feed screw mechanism coupled to the servomotor for moving one of the electrode tips with respect to the other. Workpieces to be welded to each other are sandwiched between the electrode tips. While the workpieces are being pressed by electrode tips operated by the servomotor, an electric current is passed through the workpieces to weld the workpieces to each other. The motor-driven welding gun is thus easily capable of welding workpieces even when they have a different thickness or a different number of workpieces are put together in overlapping relation.
In order to increase the strength of workpieces to be welded, it is desirable to increase the thickness of the workpieces and to overlap the workpieces in a complex configuration. It is also desirable to reduce strains produced when the workpieces are pressed. To meet these demands, it is necessary to increase the torque generated by the motor-driven welding gun.
For reducing the manufacturing cost of welded products, it is necessary to use the motor-driven welding gun at an increased ratio so as to increase a facility usage ratio for higher productivity.
When the torque and usage ratio of the motor-driven welding gun is increased, however, the amount of heat generated by the servomotor is also increased. Therefore, the servomotor needs to have an increased wall thickness and an increased size, and an add-on cooling structure needs to be mounted on the servomotor, resulting in a considerable increase in the manufacturing cost of the motor-driven welding gun.
Attempts to increase the torque of the motor-driven welding gun result in a considerable increase in the radial size of the servomotor, making the servomotor heavy. The large and heavy servomotor is liable to reduce the efficiency with which to operate the motor-driven welding gun. If the rotor of the servomotor has a larger weight and diameter, then the servomotor produces a larger centrifugal force when the rotor rotates, lowering the speed at which the electrode tips move toward and away from each other, with the result that the efficiency of the spot-welding operation is reduced.
For speeding up the welding operation, it is desirable to increase the ease with which to move the motor-driven welding gun. Such a demand can be met by reducing the weight and size of the motor-driven welding gun.
The feed screw mechanism has a feed screw coupled for corotation with the rotor of the servomotor. The surface pressure acting between the rotor and the feed screw needs to be kept at a certain level or higher in order to increase the torque of the servomotor, reduce the size of the servomotor, and prevent the rotor and the feed screw from slipping at their joint.
To satisfy the above requirements and avoid the above drawbacks, there is known a motor-combined drive unit in a motor-driven welding gun as disclosed in Japanese laid-open patent publication No. 2000-343231, for example. According to the disclosed motor-combined drive unit, a power lock mechanism based on the wedging action of inner and outer rings is disposed between a ball screw and a motor shaft. The ball screw is fixed to the motor shaft by the power lock mechanism.
With the power lock mechanism, however, the motor-combined drive unit is made up of an increased number of parts. Therefore, the motor-driven welding gun is complex in structure, and is manufactured at a considerably high cost.
When workpieces are spot-welded by the motor-driven welding gun, since a large current flows between the electrode tips, a welding gun assembly and the electrode tips are required to be forcibly cooled. The welding gun assembly and the electrode tips are cooled by cooling water which flows through cooling water passages that are incorporated in the welding gun assembly and the electrode tips.
The above cooling structure tends to cause a water leakage in the servomotor when the electrode tips are replaced or due to aging of the seal at the cooling water inlet in the welding gun assembly. When the cooling water leaks, it enters the coils in the servomotor or an encoder coupled to the servomotor, resulting in an insulation failure, an electric short circuit, or a rust-caused contact failure, which makes it difficult or impossible for the servomotor to achieve its desired performance.
Because the electrode tips of the motor-driven welding gun are opened and closed or their opening is controlled by the servomotor, if the servomotor fails to perform its controlling operation properly, then the electrode tips may fail to be opened or closed properly. Particularly, if electrode tips fail to be opened properly, a nut holding one of the electrode tips hits the rotor, applying an impact load to the joint between the rotor and the feed screw. Therefore, the rotor and the feed screw slip against each other in their joint, and tend to be damaged.